Add test mass susceptibility to the coating Brownian amplitude calculation with unit tests for non-default calculations

• Adds a tst_suscept option to coating_brownian for amplitude noise calculations so that the test mass susceptibility can be used as it is in the quantum and residual gas calculations.
• Since this noise is not computed by default and therefore not tested by our current tests with the canonical ifos, unit tests are added to test this calculation in particular.
• Some unit tests are also added for other noises that are not tested by the canonical ifos.

The following plot compares the coating Brownian calculation for A+ without amplitude noise (the current default) and with amplitude noise using the free mass and suspension susceptibilities. It's about a 3% effect for A+ at 10 Hz and becomes visibly noticeable around 4 Hz. Since it's not a slow calculation, I'm tempted to just include it by default.